The invention relates to an electrical system for use in isolation, the system comprising a main engine, such as a gas turbine or a diesel engine, a generator driven by the main engine for generating a first alternating voltage, a first rectifier unit for converting the first alternating voltage generated by the generator into a direct voltage on a DC bus, and at least one inverter unit for converting the direct voltage on the DC bus into a second alternating voltage.
In accordance with the above, the invention relates to an electrical system for use in isolation, which means, for example, an electrical system of a ship or some other station, for example one located in a desert, isolated from other electric networks. An above type of electrical system for a ship is known, for example, from European Patent 97,185. The publication teaches the use of a synchronous generator as a shaft generator driven by the shaft of the main engine, the synchronous generator supplying power to two electric networks. In one network, the voltage and frequency are determined by the voltage and frequency of the shaft generator, i.e. they are directly dependent on the rotation speed of the ship's main engine. To the other electric network, power is supplied by an AC inverter arrangement, which comprises a rectifier circuit and an inverter circuit, by which is generated an alternating voltage with a constant amplitude and frequency. The two separate electric networks are used so that the ship's primary actuators, such as motors, pumps, compressors, ventilating and cooling systems, etc. obtain the power supply directly from the network that is being fed by the synchronous generator, whereas the electric power required by radio equipment, navigation systems and lighting is obtained via the AC inverter arrangement.
The above-described known system has some defects. For example, a separate energizing circuit and a separate electric motor have to be arranged so as to be able to start the main engine. Another major problem may arise when the electricity produced by the inverter system is not necessarily of high quality, i.e. it contains harmonic waves, which are harmful in many applications. Also, in case of main engine failure, the system is also too vulnerable, if there are no auxiliary engines or arrangements by which the ship's propeller can be driven even in case of such main engine failure.
U.S. Pat. Nos. 4,883,973 and 4,330,743, respectively, teach an electrical system for a vehicle and an aircraft. In both systems, the same electric machine is used both as a motor when the main engine, i.e. the motor of the vehicle or aircraft, is started, and as a generator generating electric power to the electrical system after the motor has started. In U.S. Pat. No. 4,883,973, the motor is an asynchronous machine that is coupled to the vehicle's motor through a bimodal gear set. The gear set operates as a reduction drive during the starting mode and as a direct drive during the generating mode. When the vehicle's motor is being started, the rotary speed of the asynchronous machine has to be registered by a tachometer so as to effect feedback control in the inverter arrangement controlled by a computer-based controller feeding the asynchronous machine, so that the arrangement can maintain the desired voltage/frequency ratio in the output voltage. The motor used in the electrical aircraft system disclosed in U.S. Pat. No. 4,330,743 is a synchronous machine coupled to an aircraft engine via a drive unit containing a hydraulic torque converter. When the aircraft engine is being started, the synchronous machine is used as a motor, whereby the inverter unit feeding the synchronous machine needs data on the position of the rotor. When the aircraft engine is being started, the synchronous machine operates as a motor, and its speed is at first raised to the operating speed. After this, the hydraulic torque converter is used to accelerate the aircraft engine gradually to a desired speed so as to start the engine.
Although U.S. Pat. Nos. 4,883,973 and 4,330,743 teach the use of one and the same electric machine both as a motor when the main engine is being started and as a generator when the main engine is running, expensive and clumsy electrotechnical control arrangements are needed particularly for the use as a motor in the arrangement of both publications, the control arrangements comprising a tachometer or a rotor position transmitter, and torque converter and gear arrangements. The torque converter and gear arrangements used are such that they cannot be applied reasonably to transmission of high power, such as megawatts. Also, the operation of the electrical systems known from the two publications is based on direct voltage supply obtained, for example, from a battery. This sets certain limitations to application of these systems.